1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera.
2. Description of the Prior Art
Unlike a zoom lens system for a single lens reflex camera which requires a space for a quick-return mirror behind the photographing lens system, a zoom lens system of a compact camera does not require such a space, so that a zoom lens system for a compact camera does not need a long back focal distance. Accordingly, a telephoto-type lens system having a positive lens group and a negative lens group, in this order from the object, is generally employed in a compact camera, whereas a retrofocus-type lens system having a negative lens group and a positive lens group, in this order from the object, is employed in a single lens reflex camera.
In such a telephoto-type zoom lens system, in order to attain a zoom ratio of 3 or more, the object-side positive lens group is divided into two lens groups so that a three-lens-group lens system is obtained together with the negative rear lens group. In a three-lens-group zoom lens system, by varying the distance between the first and second lens groups, the zoom ratio of the entire lens system can be secured while field curvature is mainly corrected. Furthermore, since there are more distance parameters in a three-lens-group than in a two-lens-group arrangement, the overall length of the three-lens-group arrangement can be maintained relatively shorter with respect to a higher zoom ratio.
However, the sum of the length in the optical axis direction of each lens group, which influences the thickness of the camera body (i.e., the thickness in the optical-axis direction of the photographing lens system), has to become larger due to an increase of the number of lens elements in each lens group. Therefore even if the overall length of the lens system at the photographing position is shortened, the length of the second lens group, in particular, has a large influence on the thickness of the camera body, so that compactness of the camera is sacrificed.
The present invention provides a telephoto-type three-lens-group zoom lens system which attains (i) the angle of view of more than 50xc2x0 at the short focal length extremity, (ii) a zoom ratio of 3 or more (particularly more than 3.5), and (iii) reduces the thickness of the camera body by reducing the thickness of each lens group. Furthermore, the power of each lens group is adequately determined to reduce the overall length of the zoom lens system, so that both the overall length and the diameter of the zoom lens system are reduced as a result.
As an aspect of the present invention, there is provided a zoom lens system including a positive first lens group, a positive second lens group, and a negative third lens group, in this order from the object. Zooming is performed by moving the first, second and third lens groups in the optical axis direction. The positive first lens group is constituted by a negative lens element and a positive lens element, in this order from the object. The zoom lens system satisfies the following condition:
0.15 less than xcexa3d2G/fW less than 0.28xe2x80x83xe2x80x83(1)
wherein
xcexa3d2G designates the distance from the most object-side surface of the positive second lens group to the most image-side surface thereof; and
fW designates the focal length of the entire zoom lens system at the short focal length extremity.
The zoom lens system preferably satisfies the following condition:
1.6 less than fT/f1G less than 4xe2x80x83xe2x80x83(2)
wherein
fT designates the focal length of the entire zoom lens system at the long focal length extremity, and
f1G designates the focal length of the positive first lens group.
According to another aspect of the present invention, there is provided a zoom lens system including a positive first lens group, a positive second lens group, and a negative third lens group, in this order from the object. Zooming is performed by moving the first, second and third lens groups in the optical axis direction thereof. The positive first lens group is constituted by a negative lens element and a positive lens element, in this order from the object. The zoom lens system satisfies the following conditions:
0.15 less than xcexa3d2G/fW less than 0.33xe2x80x83xe2x80x83(1xe2x80x2)
2.0 less than fT/f1G less than 4xe2x80x83xe2x80x83(2xe2x80x2)
wherein
xcexa3d2G designates the distance from the most object-side surface of the positive second lens group to the most image-side surface thereof;
fW designates the focal length of the entire zoom lens system at the short focal length extremity;
fT designates the focal length of the entire zoom lens system at the long focal length extremity, and
f1G designates the focal length of the positive first lens group.
In either of the above-explained two aspects of the present invention, the zoom lens system preferably satisfies the following condition:
xe2x88x929 less than fT/f3G less than xe2x88x927xe2x80x83xe2x80x83(3)
wherein
fT designates the focal length of the entire zoom lens system at the long focal length extremity, and
f3G designates the focal length of the negative third lens group.
In either of the above-explained two aspects of the present invention, for the purpose of correcting chromatic aberration, it is advantageous for the positive second lens group to be constituted by cemented lens elements including a positive lens element and a negative lens element. Furthermore, if the negative third lens group is constituted by a positive lens element, a negative lens element and a positive lens element, in this order from the object, fluctuations of aberrations can be reduced over the entire focal length ranges.
The positive second lens group preferably includes a lens element having an aspherical surface satisfying the following condition:
xe2x88x9240 less than xcex94IASP less than xe2x88x9210xe2x80x83xe2x80x83(4)
wherein
xcex94IASP designates the amount of change of the spherical aberration coefficient due to the aspherical surface under the condition that the focal length at the short focal length extremity is converted to 1.0.
The negative third lens group preferably includes a lens element having an aspherical surface satisfying the following condition:
0 less than xcex94VASP less than 0.7xe2x80x83xe2x80x83(5)
wherein
xcex94VASP designates the amount of change of the distortion coefficient due to the aspherical surface under the condition that the focal length at the short focal length extremity is converted to 1.0.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-248809 (filed on Aug. 20, 2001) which is expressly incorporated herein in its entirety.